In general, a wheel loader is used for excavating a hillock using a bucket carried thereby and then placing the scooped gravel on a dump truck. FIG. 4 schematically illustrates by way of a plan view a series of operations from excavation of the wheel loader till loading of the scooped gravel on the dump truck. This represents a typical operation pattern most frequently used with the wheel loader that is called a V-shape operation.
To ultimately shorten a time required for the V-shape operation (comprising operations I to V as identified by arrow marks), it is required that the operation is performed in the following manner.
In this case, it is assumed that the automatic speed changing apparatus performs automatic speed changing from a first speed to a second speed and vice versa during forward/rearward movement of the wheel loader.
Specifically, the wheel loader a moves fast forwardly toward the hillock b at a forward second speed (F.sub.2) (operation I). When the wheel loader a comes near to the hillock b (with a distance of 0.5 m to 1.0 m), it plunges into the hillock b while the forward second speed (F.sub.2) is shifted down to a forward first speed (F.sub.1) to increase a magnitude of tractive force (operation II). On completion of the excavating operation, the wheel loader a moves rearwardly from the excavating operation site at a rearward second speed (R.sub.2) (operation III). Thereafter, the direction of running of the wheel loader a is changed to another one to run fast with the forward second speed F.sub.2 toward a dump truck c to place the excavated gravel on the dump truck c (operation IV).
On completion of the loading operation on the dump truck c, the wheel loader a moves fast at the rearward second speed R.sub.2 away from the dump truck c to return to the original position (operation V).
In this manner, when the wheel loader a performs automatic speed changing during the aforementioned operations, the wheel loader a assumes a speed stage of the second speed without fail at the time of start after changing from the forward movement to rearward movement and vice versa as well as normal running (operations I, III, IV and V). Only when the vehicle speed is reduced lower than a preset vehicle speed during the excavating operation (operation II), the second speed is shifted down to the first speed and the wheel loader a runs at this speed stage.
With the conventional speed changing apparatus, however, there arise the following drawbacks, because the speed stage is automatically shifted down from the forward second speed F.sub.2 to the forward first speed F.sub.1 corresponding to the vehicle speed.
For example, during an approaching operation of the wheel loader toward the dump truck, i.e., during the operation IV, an operator is required to displace the wheel loader forwardly at the forward second speed F.sub.2, raise up lift arms and then visually calculate a distance of approach just in front of the dump truck so as to assure that a bucket position coincides with a loading height above the ground surface for the dump truck. In practice, undesirable mistakes such as incorrect changing from the operation III to the operation IV and incorrect raising of the lift arms up to the loading height just in front of the dump truck take place due to erroneous visual calculation of a size of the loading truck and an approach distance. Further, when the wheel loader comes nearest to the dump truck, similar mistake such as incorrect raising of the bucket position up to the loading height may take place at a narrow field site under a condition that the approach distance is unavoidably shortened.
In view of these mistakes, when a distance between the wheel loader and the dump truck becomes short during the operation IV, an operator depresses a throttle pedal to increase a raising speed of the lift arms. Then, the decelerates the vehicle speed by depressing a braking pedal so as to assure that the bucket is raised up to the loading height just in front of the dump truck.
As the vehicle speed is decreased by depressing the brake pedal, the conventional apparatus is operated such that the forward second speed F.sub.2 is automatically shifted down to the forward first speed F.sub.1 when the vehicle speed is reduced lower than the preset vehicle speed.
When the forward second speed F.sub.2 is shifted down to the forward first speed F.sub.1 in that way, the vehicle speed is reduced within a very short period of time to assume a quickly braked state with the result that the load placed on the dump truck falls down and an operator may feel unpleasant. In an extreme case, a certain operation can not be accomplished smoothly.
The present invention has been made with the foregoing background in mind and its object resides in providing an automatic speed changing apparatus preferably employable for a wheel loader which assures that any shift-down does not take place in correspondence to the vehicle speed during the approaching operation toward a dump truck (operation IV).